IntroductionSolTrace is a software tool developed at the National Renewable Energy Laboratory （ NREL ） to model concentrating solar power optical systems and analyze their performance. Although originally intended for solar applications， the code can also be used to model and characterize many general optical systems. The creation of the code evolved out of a need to model more complex optical systems than could be modeled with existing tools (Jorgensen, 1991） including OPTDSH （Steele et al。， 1991） and CIRCE (Ratzel and Boughton, 1987）. SolTrace can model parabolic trough concentrators as well as dishes ， towers or other unique geometries （linear power towers, solar furnaces ， etc 。 ） . It models optical geometries as a series of stages composed of optical elements that possess an extensive variety of available attributes including shape, contour， and optical quality。 The software rapidly displays and saves data as scatter plots ， flux maps, and can output data for subsequent analysis using other software packages (e 。 g. ， Excel, etc 。 ） 。 Figure 1 shows an example of one of the output windows。The code utilizes a ray-tracing methodology （ Spencer and Murty, 1962 ） 。 The user selects a given number of rays to be traced. Each ray is traced through the system while encountering various optical interactions。 Some of these interactions are probabilistic in nature （e。g。 selection of sun angle from sun angular intensity distribution) while others are deterministic (e.g. calculation of ray intersection with an analytically described surface and resultant redirection.） Such a code has the advantage over codes based on convolution of moments in that it replicates real photon in...
